166 ECHINODEEMATA. 



fully constructed, being made up of innumerable pieces accurately fitted 

 together. The mouth occupies the centre of the ventral surface, and is 

 surrounded by radiating furrows, in which are seen minute apertures 

 that give passage to a set of remarkable prehensile organs, to be de- 

 scribed hereafter : these are calculated to act as suckers, and so disposed 

 as either to fix the body of the animal, or to retain food during the 

 process of deglutition. 



(438.) Leaving the Ophiurce, we are led through a long series of 

 almost imperceptible gradations to the Star-fishes (Asterias) (fig. 87) ; 

 in these, from the increased size of the body, the rays are united at 

 their origin, and become so much dilated as to contain prolongations of 

 the viscera lodged in their interior an arrangement not met with in 

 Ophiurce and other slender- rayed Asteridse. The dilatation of the central 

 part proceeds, and in the same proportion the rays become obliterated ; 

 so that by degrees, the asteroid shape becomes totally lost by the pro- 

 gressive filling up of the interspaces between the rays, and we arrive 

 ultimately at completely pentagonal forms, the sides of the pentagon 

 being perfectly straight lines. 



(439.) It is extremely interesting to observe the changes which occur 

 in the nature of the locomotive organs during these diversifications of 

 external figure. We have seen that, in the lower Echinodermata, pos- 

 sessing long and flexible rays, such organs were fully adequate to per- 

 form all movements needful for progression ; but as the mobility of 

 these parts is diminished by their gradual curtailment and the filling 

 up of the spaces between them, some compensating contrivance becomes 

 indispensably necessary; and accordingly we find an apparatus gradually 

 developed, well calculated to meet the exigences of the case. In Ophi- 

 ura we have already mentioned the existence of protrusible suckers 

 around the opening of the mouth, well adapted, from their position, to 

 take firm hold of food seized by the animal ; and it is by increasing the 

 number of such organs that ample compensation is made for the loss of 

 motion in the rays themselves. On examining the lower surface of an 

 Asterias, even in those forms which most approximate a right-lined 

 pentagon in their marginal contour, the number of rays will still be 

 found to be distinctly indicated by as many furrows radiating from the 

 mouth, and indicating the centre of each division of the body. These 

 " ambulacral furrows" as they are termed, exhibit, when examined in 

 a dried specimen, innumerable orifices arranged in parallel rows, through 

 each of which, when alive, the animal could protrude a prehensile 

 sucker, capable of being securely attached to any smooth surface 

 (fig. 87). 



No verbal description can at all do justice to this wonderful mecha- 

 nism, even leaving out of the question the means by which each indi- 

 vidual sucker is wielded (of this we shall speak hereafter) ; but let any 

 of our readers, when opportunity offers, pick up from the beach one 



